1. Field of the Invention
This invention relates to cable connectors and methods for making them. More specifically, this invention relates to end connectors for antenna guy cables, and in particular, to end connectors for guy cables having a fibrous aramid interior core covered with a braided metallic conductor in which the conductor is insulated from the core and the cable as a whole is insulated by an insulating sheath surrounding the conductor.
2. Brief Description of the Prior Art
Radio transmitting and receiving antennas are often large grid work structures having a height on the order of the wave length of the radio wave being transmitted or received. For a quarter wave length transmitting antenna operating in the commercial medium frequency (MF) band, an antenna having a height on the order of a hundred meters may be used. Typically, such an antenna will be a tower having a relatively small cross sectional area. The tower is supported mechanically against the effects of environmental forces by a set of guys extending radially and symmetrically to the ground or other supporting structure from various levels of the tower. Several sets of guys, each set extending from a fixed height on the tower, may be used. The guys must have substantial mechanical strength and very low stretch. Consequently, in the past, steel cables have been used. However, steel cables suffer from several disadvantages, particularly in the case of transmitting antennas. Depending on the relationship between the wavelength of the radiated wave and the length of the steel cable, a portion of the transmitted wave may be dissipated as heat in the cable or the transmitted wave may be reradiated by the cable, thus distorting the desired radiation pattern of the antenna.
Recently, cable which is practically transparent to radiowaves and which also has substantial mechanical strength and very low stretch has become available. PHILLYSTRAN (Registered Trademark of United Ropeworks (U.S.A.) Inc., Montgomeryville, PA) HPTG-TR cables are constructed of a central core rope of aramid fibers covered with an inner sheath of a synthetic plastic material, which is in turn covered by a sheath of braided metallic wire and an exterior sheath of a synthetic polymeric material (hereafter referred to a "HPTG-TR" cable). These cables are substantially lighter than steel cables and may be substituted for steel cables greatly simplifying antenna construction.
In the construction of many types of transmitting antennas, the steel guy cables must be sectioned and insulators inserted between the sections so that the sections are short in comparison with the wavelength being transmitted. In this way, the heating losses associated with the conductive sections of the cable are minimized and the resulting distortion associated with the radiating pattern of the antenna are reduced. However, such construction is expensive, in particular, when used in directional antenna systems having several towers. High frequency antennas are also sectioned, and the guys are carefully placed in an attempt to avoid the reradiation associated with the use of steel guy cables in such construction. The use of PHILLYSTRAN or other HPTG-TR-type cables eliminates the need for such sectioning.
However, in utilizing the HPTG-TR cables, there is a need for terminal connectors which permit mechanically secure connections to be made to various antenna elements. These elements include portions of the antenna tower itself, such as those which generate high electrostatic fields, as well as sections of steel cable attached at ground level. Further, there is a need for end connectors which permit the HPTG-TR cable to be protected against damage by corona discharge which may occur in areas of high electrostatic field strength, as, for example, in the vicinity of a radiating antenna. Further, when PHILLYSTRAN HPTG-TR cables are used as elements of wire antennas, the connectors must be adaptable for conducting radiofrequency waves to the cable from a transmitter source.
The present invention provides connectors and methods for forming connectors on cables which have a core comprising synthetic filaments encased in an inner sheath of braided metallic filaments and an outer sheath of synthetic polymeric material.
Cable connectors for mechanically securing the ends of such cables under both relatively high and relatively low tension conditions, and methods for making such cable connectors are taught. Further, connectors for mechanically securing the ends of such cables under high electrostatic field conditions and methods for constructing such connectors are disclosed. In addition, connectors for electrically connecting such cables to sources of radiofrequency energy and methods for making such connectors are given.
The present invention provides connectors for HPTG-TR cable which permit the cable to be mechanically secured. The connectors also permit electrical connection to be made to the conductive portion of the cable without reducing the mechanical strength of the core of the cable. The connectors also prevent the penetration of moisture into the interior of the cable. The connectors also advantageously provide contact to the conductive portion of the cable which is at least as conductive as the conductive portion of the cable itself.
The connectors also provide a means for supplying high frequency electromagnetic energy to the conductive portion of the cable so that the cable may be used in constructing high frequency radiowave transmitting antennas. The connectors also advantageously reduce the likelihood that either corona discharge or an intensive radiated electric field will damage or destroy the structural core of the cable.
Depending on the mechanical tension which is to be applied to the cable and the intensity of the electrical field to which the cable is to be subjected, the present invention takes on a number of different embodiments. For example, when it is desired to connect the end of a cable which is subject to relatively low mechanical tension and a relatively low power electrical field, a first embodiment of the present invention described below may be used. By relatively low mechanical tension is meant up to about forty percent of the breaking strength of the cable. By relatively low electrical power is meant an electrostatic field having about thirty percent of the expected corona voltage of the cable. On the other hand, when the cable is to be subject to high mechanical tension and an electrical field having a high power, the third embodiment described below may be used. By high mechanical tension is meant about one hundred percent of the breaking strength of the cable or greater. By high power electrical field is meant more than one hundred percent of the expected corona voltage of the cable.
When structural considerations dictate that the conductive portion of the cable be terminated in the presence of a high electrical field but that the insulating non-conductive core of the cable be continued, as when the cable is used as a portion of a high frequency radio transmitting antenna, the second connector embodiment described below may be used.
Amateur radio transmitting antennas may be constructed using HPTG-TR cable and the connectors of the second embodiment described below. For example, a half wave dipole antenna may be constructed by stringing a section of HPTG-TR cable of appropriate length horizontally. The outer polymeric jacket and the inner braided conductor are removed from either end and from a section of the middle of the length of HPTG-TR cable giving two conductive sections. For example, for an antenna transmitting at about 29 megahertz, the total length of the radiating dipole including both of the conductive sections must be about five meters. Both of the conductive sections are terminated at either end using connectors of the second embodiment described below Clamps are attached to both of the interior connectors and are used to feed the antenna with radiofrequency energy. The nonconductive core at the ends of the HPTG-TR cable may be connected with lengths of another nonconductive structural rope or cable, such as nylon rope, to support the antenna.
In general, the electrically conductive portions of the connectors described herein are formed from metal, preferably aluminum. However, other electrically conductive materials may be employed. Aluminum is preferred because the metallic braided inner conductive sheath of PHILLYSTRAN HPTG-TR cable is aluminum, and consequently the same material is preferred as it has the same conductivity as the inner conductive sheath, junction potentials are avoided, etc. Other conductive materials, such as copper, or stainless steel may also be used. However, the material must be chosen so that the electrical conductivity and mechanical strength of the connector formed are not significantly diminished.
Similarly, the various members used to expand the inner sheath of braided metallic filaments and to expand the filaments of the aramid core may be made from aluminum. Expansion members used to expand the core filaments are preferably solids, such as solid aluminum. The electrically conductive clamp used to electrically connect the braided metallic inner sheath may be stainless steel.